An effective electrical power cable needs to satisfy several competing structural needs. On one hand, a power cable should be lightweight, easy to handle, and inexpensive to produce. On the other hand, a cable should be solidly built, exhibit good fire retardancy properties (if required), and be rigid enough to withstand the rigors of the elements and the stresses placed on it during installation. Maximizing any one of these characteristics, however, often has a detrimental impact on at least one of the others. Moreover, nonfunctional features such as the surface finish of the completed cable often play a factor in the acceptance level of a power cable. Consequently, existing power cables, such as the cable depicted in FIGS. 1 and 2, typically strike a compromise between these needs.
FIG. 1 is a transverse cross-sectional view of an exemplary conventional cable. The cable contains three “cores,” with each core being a semi-finite structure comprising a conductive element 105 and at least one layer of electrical insulation 120 placed in a position radially external to the conductive element 105. When considering a cable for medium voltage electrical power, the core may also comprise an internal semiconductive covering 115 located in a position radially external to the conductive element, an external semiconductive covering located in a position radially external to the layer of electrical insulation 125, and a metal screen in a position radially external to the external semiconductive covering (not shown).
For the purposes of the present description, the term “multipolar cable” means a cable provided with at least a pair of cores as defined above. In greater detail, if the multipolar cable has a number of cores equal to two, the cable is technically termed a “bipolar cable,” and if the cores number three the cable is known as a “tripolar cable.” The conventional cable of FIG. 1 is a tripolar cable.
The cores, along with ground wires 110, are joined together to form a so-called “assembled element.” Preferably, the joining is accomplished by helicoidally winding the cores and ground wires together at a predetermined pitch. As a result of the joining and winding of the cores, the assembled element has a plurality of interstitial zones 130, which are defined by the spaces between the cores and ground wires. In other words, the joining and winding of the cores and their circular shape gives rise to a plurality of voids between them.
The production process for a conventional multipolar cable comprises the step of filling the interstitial zones 130 to confer a circular shape to the assembled element. The interstitial zones, which are also known as “star areas,” are generally filled with a filler of the conventional type (e.g., a polymeric material applied by extrusion). The resulting circular shape provides a solid body with a symmetrical appearance and feel.
The cable is finished by applying at least one other layer, the nature of which, as well as the number of layers, depend on the type of multipolar cable to be obtained. In the conventional cable of FIG. 1, a layer of binder tape 135 may be provided in a position radially external to the assembled element, and a polymeric inner jacket layer 140 is provided in a position radially external to the binder tape. This inner jacket layer 140 is typically made from a polymeric material and is extruded over the binder tape. Given the circular cross-section of the assembled element, inner jacket layer 140 assumes the shape of the binder material or filling material, i.e., the inner jacket also becomes circular in cross-section. Finally, a metallic armor 145 is provided in a position radially external to the inner jacket layer 140, and the entire cable is clad in a polymeric outer jacket 150.
FIG. 2 is a longitudinal perspective view of the conventional cable of FIG. 1. The same numbering has been used as in FIG. 1 to show the correlation between the drawings. FIG. 2 illustrates the concentricity provided by the filling material 130 in the voids around and between the conductive elements 105.
This type of conventional cable has historically been employed in industrial and commercial power cable applications (e.g., installation in cable trays, troughs, and ladders) as a replacement for cable enclosed in metal conduit and certain classifications of hazardous locations as defined by local codes and authorities. For combustible hazardous environments, the outer jacket of the cable often comprises fire retardant polymers. These cables comply with nationally regulated flame retardancy tests, such as defined in the standards IEEE-1202 (“Standard for IEEE Standard for Flame Testing of Cables for Use in Cable Tray in Industrial and Commercial Occupancies”), UL-1685 (“Standard for Vertical Tray Fire Propagation and Smoke Release Test for Electrical and Optical Fiber Cables”), CSA Std. C22.2 FT-4 (vertical flame test), and IEC 332-3 (vertical-tray, high-energy combustion propagation test) specifications. For example, to satisfy the requirements of CSA Std. C22.2 FT-4, the cable is subjected to a burner mounted 20° from the horizontal with the burner facing up. To pass the test, the cable may only char within 1.5 m of the burner. The other standards require subjecting the cable to similar fire retardancy tests.
For a number of reasons (e.g., weight reduction), expanded polymeric materials have been used for the conventional filler and jacketing materials. Expanded polymeric materials are polymers that have a reduced density because gas has been introduced to the polymer while in a plasticized or molten state. This gas, which can be introduced chemically or physically, produces bubbles within the material, resulting in voids. A material containing these voids generally exhibits such desirable properties as reduced weight and the ability to provide more uniform cushioning than a material without the voids. The addition of a large amount of gas results in a much lighter material, but the addition of too much gas can negatively impact the surface finish of the polymer and decrease some of the resiliency of the material.
The expanded material is typically extruded to form its desired shape. After the material leaves the extrusion die, it stretches and cools. The degree of stretching is defined by the drawdown ratio. More specifically, the drawdown ratio is calculated as the ratio of the cross-sectional area of the material as it leaves the extrusion die to the material's cross-sectional area after cooling. Applicants have recognized that controlling the drawdown ratio can help achieve a relatively high degree of expansion while also maintaining required resiliency and achieving a smooth surface finish.
Several publications describe power cables that include expanded materials. For example, WO 02/45100 A1 discloses a modified conventional cable using an expanded material as a filler between the interstitial areas created in the assembled element. The use of expanded material as a filler results in a cable that is lighter than the conventional cable and provides improved impact resistance. But due to the somewhat unpredictable expansion of the filler disclosed in that publication, a containment layer is required to achieve a substantially circular cable. This layer requires further processing, adding to the overall cost of the cable.
U.S. Patent Application Publication 2003/0079903 A1 discloses a cable wherein both the outer jacket and the filled interstitial zones may contain expanded material. This cable is allegedly lighter than the cable of WO 02/45100 A1. U.S. Pat. No. 6,501,027 B1 and U.S. Patent Application Publication 2003/0141097 A1 disclose multipolar cables with a layer of expanded polymeric material in the outer jacket.
Although these documents address the use of expanded materials particularly in the outer jackets of electrical power cables, Applicants have noted that the interior structure of the cable provides opportunities to decrease cable weight while maintaining the required structural characteristics. Furthermore, Applicants have recognized that when a metal protection is used in the cable structure such as a metallic armor, in particular in multipolar cable designs, the use of an expanded material layer inside the metal protection provides additional protection. For example, in case an impact causes a permanent deformation of the metal protection, an inner expanded layer may protect what might otherwise result in a compression of the insulation of one or more of the cores enclosed within the metal protection, thereby resulting in a reduced electrical stress resistance capability when the cable is under load. In addition, Applicants have recognized that balancing the expansion degree and drawdown ratio of the manufacturing process for expanded materials can lead to lighter power cables with satisfactory impact resistance and cosmetic finish.